A Kentucky doctor who was dragged from an overcrowded United Airlines plane remained hospitalized Tuesday in Chicago where he was "undergoing treatment for his injuries," according to a family statement.

There is no indication that anything in Dao's personal or professional history factored into the onboard incident, but an investigative team from WLS-TV, ABC7's sister station, has learned more about the man who was dragged from a flight on Sunday.

Video shows three security officials dragging a male passenger from a United Airlines flight that the airline said was overbooked as it waited to depart from Chicago's O'Hare International Airport. (Courtesy Jayse Anspach/Twitter)

Kentucky licensing officials only allow Dao to see patients once a week, following a series of criminal and regulatory actions against him the past 14 years.

In 2003, he was charged with 98 counts of illegally prescribing and trafficking prescription painkillers.

Dao, a native of Saigon, Vietnam, where he attended medical school, came to the U.S. in 1980. He had a private practice in rural Kentucky for 17 years until his arrest in 2003 for unauthorized prescribing of controlled substances in connection with a sexual relationship he was having with a former patient.

At the time of his arrest, Dao tested positive for pain meds and his hospital privileges were suspended.

Twitter users mocked United Airlines for it's handling of an overbooked flight incident.

Dao was convicted by a jury in Jefferson County, Kentucky in November 2004 on six felony counts and sentenced to five years of supervised probation.

The United incident may not have been Dao's first brush with force.

Dao was quoted by a culinary school newspaper in 2004 as saying that an inmate at Indiana State Prison in Michigan City once tried to strangle him with his own stethoscope. He was quoted as saying the incident occurred in 1980 while he was working as a prison doctor.

Officials at the state of Indiana Department of Corrections said they have no record of Dao ever as a state employee but that there may have been outside medical contractors working in the prison at the time.

The incident was started when Dao was asked to leave the plane to accommodate United employees who needed to get to Louisville, Dao declined because he needed to see patients Monday morning, other passengers said.

As the flight waited to depart from O'Hare, officers could be seen on cellphone video grabbing the screaming Dao from a window seat, pulling him across the armrest and dragging him down the aisle by his arms. The airline was trying to make room for four of its employees on the Sunday evening flight to Louisville, Kentucky.

Dao's family issued a statement through Chicago attorney Stephen L. Golan, which said they are "very appreciative of the outpouring of prayers, concern and support they have received. Currently, they are focused only on Dr. Dao's medical care and treatment."

The family is also being represented by Chicago aviation attorney Thomas A. Demetrio.